#if NETSTANDARD
namespace System;

/// <summary>
/// Represent a type can be used to index a collection either from the start or the end.
/// </summary>
/// <remarks>
/// Index is used by the C# compiler to support the new index syntax
/// <code>
/// int[] someArray = new int[5] { 1, 2, 3, 4, 5 } ;
/// int lastElement = someArray[^1]; // lastElement = 5
/// </code>
/// </remarks>
internal readonly struct Index : IEquatable<Index>
{
	private readonly int _value;

	/// <summary>
	/// Construct an Index using a value and indicating if the index is from the start or from the end.
	/// </summary>
	/// <param name="value">The index value. it has to be zero or positive number.</param>
	/// <param name="fromEnd">Indicating if the index is from the start or from the end.</param>
	/// <remarks>
	/// If the Index constructed from the end, index value 1 means pointing at the last element
	/// and index value 0 means pointing at beyond last element.
	/// </remarks>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public Index(int value, bool fromEnd = false)
	{
		if (value < 0)
		{
			throw new ArgumentOutOfRangeException(nameof(value));
		}

		_value = fromEnd ? ~value : value;
	}

	// The following private constructors mainly created for performance reason to avoid the checks
	private Index(int value) => _value = value;


	/// <summary>Create an Index pointing at first element.</summary>
	public static Index Start => new(0);

	/// <summary>Create an Index pointing at beyond last element.</summary>
	public static Index End => new(~0);

	/// <summary>Returns the index value.</summary>
	public int Value => _value < 0 ? ~_value : _value;

	/// <summary>Indicates whether the index is from the start or the end.</summary>
	public bool IsFromEnd => _value < 0;


	/// <summary>Create an Index from the start at the position indicated by the value.</summary>
	/// <param name="value">The index value from the start.</param>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public static Index FromStart(int value)
	{
		if (value < 0)
		{
			throw new ArgumentOutOfRangeException(nameof(value));
		}

		return new Index(value);
	}

	/// <summary>Create an Index from the end at the position indicated by the value.</summary>
	/// <param name="value">The index value from the end.</param>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public static Index FromEnd(int value)
	{
		if (value < 0)
		{
			throw new ArgumentOutOfRangeException(nameof(value));
		}

		return new Index(~value);
	}

	/// <summary>Calculate the offset from the start using the giving collection length.</summary>
	/// <param name="length">The length of the collection that the Index will be used with. length has to be a positive value</param>
	/// <remarks>
	/// For performance reason, we don't validate the input length parameter and the returned offset value against negative values.
	/// we don't validate either the returned offset is greater than the input length.
	/// It is expected Index will be used with collections which always have non negative length/count. If the returned offset is negative and
	/// then used to index a collection will get out of range exception which will be same affect as the validation.
	/// </remarks>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public int GetOffset(int length)
	{
		var offset = _value;
		if (IsFromEnd)
		{
			// offset = length - (~value)
			// offset = length + (~(~value) + 1)
			// offset = length + value + 1

			offset += length + 1;
		}
		return offset;
	}

	/// <summary>Indicates whether the current Index object is equal to another object of the same type.</summary>
	/// <param name="value">An object to compare with this object</param>
	public override bool Equals(object? value) => value is Index comparer && _value == comparer._value;

	/// <summary>Indicates whether the current Index object is equal to another Index object.</summary>
	/// <param name="other">An object to compare with this object</param>
	public bool Equals(Index other) => _value == other._value;

	/// <summary>Returns the hash code for this instance.</summary>
	public override int GetHashCode() => _value;

	/// <summary>Converts the value of the current Index object to its equivalent string representation.</summary>
	public override string ToString() => IsFromEnd ? ToStringFromEnd() : ((uint)Value).ToString();

	private string ToStringFromEnd() => $"^{Value}";


	/// <summary>Converts integer number to an Index.</summary>
	public static implicit operator Index(int value) => FromStart(value);
}

#endif